The present invention relates to amine glyoxal resins, and more specifically to methods of preparing amine glyoxal resins from aromatic diamines and glyoxal, and films therefrom.
U.S. Pat. No. 9,243,107 describes the synthesis of poly(1,3,5-hexahydro-1,3,5-triazine)s (PHTs) by condensation polymerization of aromatic diamines with paraformaldehyde (PF). PHTs possess a number of attractive characteristics: high Young's modulus, solvent and environmental stress crack resistance, design flexibility, and recyclability.
Typically, solid 4,4′-oxydianiline (ODA) and solid PF (1.25 molar equivalents relative to ODA based on formaldehyde molecular mass, are mixed in N-methylpyrrolidone (NMP) at 50° C. in a closed vessel. After about 15-20 minutes, the solution becomes clear, suggesting complete solubilization of ODA and cracking of PF. The solution also has a low viscosity comparable to the NMP solvent. Prolonged reaction times (about 1 hour for a 16 wt % solid composition) at 50° C. lead to a sudden (within tens of seconds) and large increase in viscosity, eventually resulting in gelation of the solution. At this stage, the gel network is referred to as a hemiaminal dynamic covalent network (HDCN), shown in Scheme 1.

Before the gelation stage is reached, the low viscosity solution can be cast into films. Subsequent curing of the film at about 200° C. results in the transformation of the network into a PHT (Scheme 1). Although this method allows for the preparation of defect-free films on a small scale, the processing time-window and low viscosity of the solution limit the range of applications (e.g., composites, coatings) of these materials. Moreover, ODA-based PHTs exhibit limited stability at temperatures above 200° C. Therefore, complete removal of the bound NMP solvent is difficult and impacts the reproducibility of film properties (e.g., thermal properties). Also, even though formaldehyde is widely used for maleimide, urea-formaldehyde and phenol-formaldehyde resins, it is highly toxic and classified as a human carcinogen. Therefore, the potential for exposure to formaldehyde during synthesis of the PHT resin and the possibility of formaldehyde leaching from the resin after film preparation are major drawbacks of PHT materials.
A need exists for a solvent-resistant resin having good thermo-mechanical and recyclability properties similar to PHTs that has a low toxicity risk during and after synthesis, as well as greater batch-to-batch reproducibility.